This invention relates to a nuclear reactor containment structure and more particularly to an over and under type nuclear reactor containment structure provided with a diaphragm floor aiming at great increase in its strength at an earthquake.
The over and under type nuclear reactor containment structure includes a diaphragm floor for dividing a closure casing into a drywell and a pressure suppression chamber and the diaphragm floor being provided with a plurality of bent downcomer passing therethrough.
A conventional diaphragm floor is arranged such that its inner end is supported by a pedestal for the pressure vessel, a plurality of radial beams of H-shaped structural steel radially extending from the pedestal and a plurality of lateral beams disposed between the radial beams being provided to form a netlike construction, each of the radial and lateral beams being back coated by a steel plate layer on which a concrete layer is formed through stud bolts, and reinforcing steels protruding radially and inwardly from the inner peripheral end of the concrete layer being embedded into the pedestal to connect the concrete layer to the latter.
Each of the radial beams is supported at its portion near the outer end by a vertical column extending from a bottom portion of the pressure suppression chamber.
The outer end of each of the radial beams is supported by a shear key provided on the inner surface of the closure casing. The shear key is adapted to permit the vertical and radial deformations of the associated radial beam, but restrain the deformation in the circumferential direction of the diaphragm floor.
The diaphragm floor so arranged in the conventional nuclear reactor containment structure acts in such a manner that since the reinforcing steels connect the concrete layer of the diaphragm floor to the pedestal, the horizontal force caused by an earthquake is transmitted from the pedestal through the reinforcing steels to the concrete layer and further through the stud bolts to the radial beams positioned below the concrete layer, and absorbed by the closure casing through the shear keys abutting against the outer ends of the radial beams.
With such an arrangement that the horizontal force is transmitted from the pedestal to the concrete layer and hence to the radial beams positioned therebelow, there are defects in respect of the strength of the reinforcing steels at the connection of the concrete layer and pedestal, the strength of the stud bolts at the connection of the concrete layer and radial beams, and the torsion of the closure casing and diaphragm floor due to the bending moment caused by the difference in the height of the concrete layer and radial beams when the horizontal force transmits through the shear keys.
These problems are serious, especially when a great earthquake occurs.